Mercury analysis system and method

ABSTRACT

The system and method utilize a vessel, such as a test tube, having a predetermined interior side surface area coated with gold. The closed end of the tube is uncoated. Mercury from a fluid sample is adsorbed by the gold coating, such as by passing a gas through the tube or by placing a liquid in the tube and shaking the tube for a short period of time. After mercury from a given amount of sample has been adsorbed by the gold coating, the test tube is placed in an inverted generally upright position with the closed end of the tube above the open end and with the open end vented. Then the tube is rapidly heated to a temperature in excess of 400* centigrade to vaporize the mercury from the gold coating. The mercury vapor will rise in the test tube to the closed end thereof and form a bubble of mercury vapor mixed with hot air. Ultraviolet light in the 2537 angstrom wave length band is passed endwise through the test tube. Some of the ultraviolet light is absorbed by the mercury vapor in the test tube and the transmitted light is detected and measured. Next, the mercury vapor laden air is flushed from the test tube with mercury free air. Ultraviolet light is again passed through the test tube, this time through mercury-free air and the amount of light transmitted through the mercury-free air is detected and measured. The difference between the light transmission measurements is indicative of the concentration of mercury in the fluid sample. In addition to the test tube, the system includes structure for carrying out the method steps described above.

United States Patent 191 Grengg MERCURY ANALYSIS SYSTEM AND METHOD [75]Inventor:

Walter M. Grengg, Madison, Wis.

Pollution Control Technology, Inc., Madison, Ill.

Aug. 11, 1972 App]. No.: 279,820

Assignee:

[22] Filed:

References Cited UNITED STATES PATENTS 3,173,016 3/1965 Williston an.250/218 3,178,572 4/1965 Williston 3,544,789 12/1970 Wieder 250/373Primary Examiner.lames W. Lawrence Assistant Examiner-C. E. ChurchAttorney, Agent, or Firm-Silverman & Cass,-Ltd.

[57] ABSTRACT The system and method utilize a vessel, such as a testtube, having a predetermined interior side surface area coated withgold. The closed end of the tube is uncoated. Mercury from a fluidsample is'adsorbed by the gold coating, such as by passinga gas throughthe tube or by placing a liquid in the tube and shaking the tube for ashort period of time. After mercury from a given amount of sample hasbeen adsorbed by the gold coating, the test tube is placed in aninverted generally upright position with the closed end of the tubeabove the open end and with the open end vented. Then the tube israpidly heated to a temperature in excess of 400 Centigrade to vaporizethe mercury from the gold coating. The mercury vapor will rise in thetest tube to the closed end thereof and form a bubble of mercury vapormixed with hot air. Ultraviolet light in the 2537 angstrom wave lengthband is passed endwise through the test tube. Some of the ultravioletlight is absorbed by the mercury vapor in the test tube and thetransmitted light is detected and measured. .Next, the mercury vaporladen air is flushed from thetest tube with mercury free air.Ultraviolet light is again passed through the test tube, this timethrough mercury-free air and the amount of light transmitted through themercury-free air is detected and measured. The difference between thelight transmission measurements is indicative of the concentration ofmercury in the fluid sample.

In addition to the test tube, the system includes structure for carryingout the method steps described above.

33 Claims, 9 Drawing Figures I PATENTELDEB 3:974

' sum 10? s PATENTEL DE!) 31974 SHEET 2 BF 3 FIG. 7

us V

2 MEMORY I22 M I I B AMPLIFIER a r LOGARITHMIC RY f CONVERTER STOREDMULT-IPLICAND MERCURY ANALYSIS SYSTEM AND METHOD BACKGROUND AND SUMMARYOF THE INVENTION The present invention relates to a method and systemfor analyzing the mercury concentration in a fluid sample which may beeither a gas or a liquid. The field of the invention includes chemicalanalysis methods and systems, particularly methods and systems whichoptically measure ray energy which passes through a fluent material.Methods and systems of this type have been classified at this time inClass 23, subclasses 232 and' 253, and Class 250, subclass 218.

More specifically, the present invention relates to a method and systemwhich include (a) adsorbing mercury in a noble metal from a fluidsample, (b) vaporizing the mercury from the noble metal by heating thesame, (c) passing ultraviolet light in the mercury vapor adsorption wavelength band of 2537 angstroms through the mercury vapor, and (d)measuring the amount of ultraviolet light absorbed by the mercury vaporto obtain an indication of the concentration of mercury in the fluidsample.

The technique of measuring the concentration of a gas or vapor bypassing radiant energy or light having a specific discrete wavelengthband which is absorbed by the particular gas or vapor being studied, iswell known. Examples of such procedures or techniques are disclosed inthe following US patents:

US. Patent 3,174,037

US. Patent 3,589,814

US. Patent 3,589,868

Also, procedures have heretofore been proposed for measuring theconcentration of mercury vapor in a gas sample such as air by passingradiant energy in the 2537 angstrom wavelength band through gas beinganalyzed, and then measuring the amount of radiant energy absorbed bythe gas, theamount of absorbed radiant energy being related to theconcentration of mercury vapor in the gas being analyzed. In thisrespect reference may be had to US. Pat. No. 3,173,016 issued to S. H.Williston et al. on Mar. 9, 1965 and to Trace Mercury Determination byJ. B. Brooks and W. E. Wolfram which appeared in Chemical & EngineeringNews, Volume 48, Page 37 (June 22, 1970).

One of the disadvantages incurred with previously proposed methods andsystems for mercury analysis, is the problem of loss of mercury from thesample and contamination of the sample (carryover) when measuring minuteamounts of mercury. As for losses, it is well known that mercury isadsorbed by many different elements and in systems where the samplebeing analyzed is transferred through tubing or otherwise from onechamber or container to another chamber or container, there is danger ofloss of mercury from the sample by reason of the mercury being adsorbedin the walls of the vessel, container or tubing by which it istransferred. Y

As for contamination, it will be understood that afte adsorbing mercury,the transfer vessel, container or tubing may later give off mercury toasucceeding fluid sample and thereby cause an erroneous measurement ofthe concentration of mercury in the succeeding sam- I ple. Theseproblems of losses and contamination are mercury are being measured suchas concentrations in the order of 0.05 parts per billion (PPB).

Another problem incurred with known methods and systems for analyzingmercury concentration in fluid samples by passing ultraviolet lightthrough the sample or through mercury vapors obtained from the samples,is brought about by the fact that the gaseous medium containing themercury vapor may also contain other gases or vapors such as organicvapors which also ab sorb radiant energy in the band width of radiantenergy absorbed by mercury. When this occurs, an erroneous measurementindicating a larger concentration of mer-' cury than actually exists,will be obtained.

Still another disadvantage incurred with prior art mercury analysissystems and methods is the fact that for measuring very smallconcentrations of mercury in a fluid sample, a large quantity of thesample must be analyzed and this takes a significant amount of time. Itis desirable, therefore, to providea method and system for analyzingmercury in which the time period for making the analysis is relativelyshort.

The method and system of the present invention overcome many of thedisadvantages incurred with prior art mercury analysis systems andmethods by eliminating transfer means such as vessels, containers ortubings, and by eliminating losses between the collection of mercuryfrom a sample and the analysis of the mercury collected. The method andsystem of the present invention are also very versatile and enable onequickly to make accurate determinations of minute concentrations ofmercury in liquids, gases and solids. The method andsystem of theinvention achieve the aforementioned advantages by utilizing one vesselhaving a predetermined interior surface area thereof coated with a noblemetal, such as gold, for the collecting vessel, the vaporizing vesseland the mercury vapor absorption vessel. Heretofore, the use of avessel, such as a test tube, lined with gold has been proposed but foran altogether different purpose, namely, to provide the interior surfaceof the tube with an essentially chemically inert lining. In this respectreference may be had to US. Pat. No. 3,475,131.

According to the invention, there is provided for use in a mercuryanalysis system, a test tube having an open end adapted to be stoppered,an interior side surface area coated with a noble metal, and an uncoatedclosed end.

Also, according to the invention, there is provided a method foranalyzing the concentration of mercury in a fluid including the steps ofplacing a given amount of fluid sample in a vessel having a noble-metalcoating therein, removing the sample for the vessel, positioning thevessel in an inverted generally upright position with the closed endabove the open end and with the open end vented, heating the vessel to agiven temperature to release mercury vapor from the noble metal coating,passing ultraviolet light of awave-length band which is absorbed bymercury vapor endwise through the vessel, measuring the amount of lighttransmitted through the mercury vapor laden air within the vessel,flushing the mercury vapor laden air from the vessel with mercury freeair, again passing ultraviolet light endwise through the vessel nowhaving mercury-free air therein, measuring the amount of lighttransmitted through the mercuryfree air, and comparing the'first lighttransmission measurement with the second light transmission measurement,the difference between the measurements being indicative of theconcentration of mercury in the fluid sample.

Further according to the invention there is provided a mercury analysissystem including a vessel supported in a generally upright position witha first end of the vessel above a second end and with the second endvented, a mechanism for .vaporizing mercury stored in the noble metalcoating on the interior surface of the vessel, and apparatus foranalyzing the mercury vapor, the analyzing apparatus including a sourceof ultraviolet light which is positioned to pass ultraviolet lightendwise through the vessel, a detector for detecting the amount of lighttransmitted through the vessel, and a mechanism for flushing mercuryvapor laden air from the test tube. Preferably the mercury analysissystem also includes electronic circuitry for converting the signalsgenerated by the detector into signal values directly related to thenumerical concentration of mercury in the fluid sample in parts perbillion (PPB).

Further, according to the invention there is provided for use in amercury analysis system a device for sampling a gas including a vesselpositioned to receive a gas therein, a suctionmechanism for drawing agas through the vessel and a'control mechanism for controlling the rateof flow and the time period of flow of the gas through the vessel.

' BRIEF DESCRIPTION'OF THE DRAWlNGS 7 FIG. I is a sectional elevationalview of the test tube of the invention.

FIG. 2 is aperspective view of a device for obtaining multiple samplesof a gas by drawing the gas through FIG. 6 is a fragmentary sectionalelevational view DESCRIPTION'OF THE PREFERRED EMBODIMENTS In FIG. 1 isillustrated a test tube 10 made in accordance with the teachings of thepresent invention. The test tube 10 is basically of conventional designhaving a closed generally.semi-spherical end 12, a generally cylindricalelongate side wall 14,and an opened end 16 defined by an outwardlyflared flange 18 extending from the cylindrical side wall 14. Accordingto the in,- vention a portion of interior side-surface 20 of the tube 10is coated with a noble metal 22. Any one of the noble metals which havea strong afinity for mercury can be utilized for the noble metal coating22. One

noble metal which has worked very well is gold, and hereinafter thenoble metal coating 22 will be simply referred to as the gold coating22.

The gold coating 22 extends along the length of the side wall I4starting at a point at or adjacent the junction between the closed end12 and the side wall 14, and extending toward the open end 16. Empiricaltests have shown that good results are obtained when the ratio of thelength of the gold coating 22 to the length of the tube 10 is a numberequal to approximately 0.4. The limitations on the length of the goldcoating 22 will be explained in greater detail in connection with thedescription of FIG. 6.

' The gold coating or film 22 on the interior surface 20 of the tube 10can be applied by several methods which are as follows: (a) high vacuumvapor deposition; (b) ionic sputtering in a low pressure inert gaselectrical discharge; (c) chemical reduction of a solution initiallycontaining the metal in soluble form; and (d) thermal decomposition of ametal-organic compound which has been applied to the inside by paintinga solution of the metal-organic compound in an organic solvent on theinterior surface 20 of the test tube 10, drying the solvent, and thenconverting the remaining film to metallic gold (or other noble metal) byheating the test tube 10 above the decomposition temperature of themetalorganic compound.

To facilitate heating of the test tube for purposes to I be describedhereinafter, outer surface 24 of the test tube 10 preferably has a blackcoating thereon. Such a black coating can consist of a black oxidecoating which is applied to the outer surface 24 after the outer surface24 has been sandblast roughened. After being applied, the black oxidecoating is fused in place.

According to the teachings of the invention, the test tube is preferablymade from a glass which transmits infra-red radiation and ultravioletlight radiation. In this respect the test tube 10 is preferably'madefrom high'silica 2537 angstrom transmitting glass. I

The flared flange 18 at the open end 16 facilitates the insertion of astopper 26 in the tube 10 as shown in FIG. 4. However, for practicingthe method or for utilizing the system of theinvention, a flared openend is not essential. r

The test tube 10 with the gold coating 22 enables one to utilize a verysimple and accurate procedure for analyzing the concentration of mercuryin a sample. Briefly this procedure involves the placing of a sample, orthe passing of a given amount of sample through the test tube 10 in sucha way that at least a portion, if not substantially all of the mercuryin the sample, is adsorbed by the gold coating 22. Then the sample isremoved from the tube 10 and the tube 10 is stoppered as shown in FIG.4. It is now ready for making an analysis of the mercury in the sampleby analyzing the mercury adsorbed by the gold'coating 22. This isaccomplished by rapidly heating the test tube 10 to vaporize the mercuryfrom the gold coating 22. Ultraviolet light in the mercury absorptionband width, namely, 2537 angstrom ultraviolet light is passed axiallythrough the tube and the amount of light which is transmitted throughthe tube is detected and compared with a measurement of light throughthe tube with mercury-free air therein. The difference in the lighttransmission measurements is indicative of the concentration of mercuryin the sample.

As will be described hereinafter, the sample can be a liquid or gas. Adevice 30 is shown in FIG. 2 for sampling a gas. The device 30 includesa housing 32 having a plurality of openings or ports 34 which extendthrough the upper surface of the housing 32. In FIG. 2, the housing 32has eight such ports 34. Each of the ports 34 is adapted to receive oneof the test tubes in inverted position, that is to say, with the closedend 12 up, as shown in FIG. 2.

Positioned within each one of the openings 34 is a pipe 36, a portion ofwhich extends above the upper surface of the housing 32 such that anupper end 38 of each pipe 36 extends into a test tube 10 received in theassociated opening 34 and opens at the upper end 38 into the test tubeadjacent the closed end 12 of the test tube 10. In this way gas samplewhich'is injected into the tube 10 through the pipe 36 will enter thetest tube 10 adjacent the closed end 12 and flow downwardly therefromalong and past the gold coating 22 in the test tube 10 as best shown inFIG. 3. It will be appreciated that by injecting the gas sample into thetest tube 10 with the pipe 36 one is assured that substantially all ofthe gas sample will flow past the gold coating 22 on the inner surfaceof the test tube 10. i

In FIG. 3 is illustrated the structure within the housing 32 beneath oneof the openings 34. This structure for receiving and holding a test tubein proper location relative to the pipe 36 includes a block 40. Theblock 40 has a top side 41 and a bottom side 43 with an inlet opening 44opening onto the bottom side 43 and a cavity 46 which opens onto the topside 41. Extending between and communicating with the cavity 46 and theinlet opening 44 is a passageway 48 in which an inner end 50 of the pipe36 is received and fixed in place. As

shown in FIG. 3 the block 40 has a spigot 52 and an outlet opening orpassageway 54 .which extends through the spigot 52. The spigot 52 has avalve member 56 mounted therein for controlling the flow of gas throughthe outlet passageway 54. A constriction 58 between the cavity 46 andthe passageway 54' limits the rate of flow when the valve member 56 isfully open. As shown in FIG. 3, the inverted test tube 10 is supportedon the block 40 with the flange 18 resting on the top side 41 and withthe open end 16 of the test tube 10 positioned over the cavity 46. Tofacilitate proper locating of the test tube 10 over the cavity 46, atubular guide member or pipe 64 is positioned in and extends outwardlyfrom the cavity 46. In this respect one end 66 of the pipe 64 isreceived and fixed in the cavity 46 with the remainder of the pipe 64extending outwardly from the cavity 46. The pipe 64 is shorter in lengththan the pipe 36 such that an outer end 68 of the pipe 64 is locatedbeneath the upper end 38 of the pipe 36 and beneath the gold coating 22in the test tube 10 when the test tube 10 is positioned as shown in FIG.3. Thus, the outer pipe 64 serves as a means for supporting and holdingthe test tube 10 in a desired location with the upper end 38 of theinner, smaller pipe coaxial with the test tube 10.

Since it is not necessary to eject gas toward the closed end 12 of thetest tube 10, the upper end 38 of the pipe 36 ejects air through portsor holes 70 in the side walls of the pipe adjacent the upper end 38 asshown in FIGS. 2 and 3. In this way a gas is injected into the test tube10 radially outwardly from the center of the test tube 10 toward thegold coating 22 on the interior surface 20 of the test tube 10.

The cavity 46 and associated openings located beneath each housingopening 34 can be formed in an individual block or in a large blockwhich contains a plurality of cavities and associated openings.

To minimize undesired adsorption and crosscontamination between samples,it is desirable to coat the gas conducting channels, such as theinterior of the pipe 36, which carry the gas to be analyzed into thetest tube 10 with a material which has a very low adsorption of mercury.It has been discovered that fluorocarbon plastic such as plastic soldunder the trademark Teflon is one such material which adsorbs verylittle mercury.

Thus, according to the teachings of the invention the fluid conductingchannels in a mercury analysis apparatus, e.g., in the sampling device30, are coated with or made of a fluoro-carbon plastic such as Teflon.

The device 30 is particularly adapted for sampling ambient air bydrawing air from the underside of the housing 32 into the opening 44through the pipe 36 and into the interior of the test tube 10. This isaccomplished by applying a suction to the outlet passageway 54. The rateof flow of air drawn through the test tube 10 is determined by the sizeof the flow limiting cons triction 58 and the partial vacuum in theoutlet passageway 54. The initiation and termination of the sample airflow is provided by a 90 rotation of the valve the test tubes 10 in oneof the openings 34 and over the" outer pipe 64 within the housing 32..Then the operator sets the appropriate timer 74 to initiate the knownrate of air flow at the desired time, arid to terminate the air flowafter a predetermined interval.

It will be understood that since theair or other gas which is sampled ispassed through the test tube 10, not

all of the mercury in the gas or air is adsorbed by the gold coating 22.However, it has been found that for a particular gas, a specificpercentage of mercury will be adsorbed by the gold coating 22 from thegas depending upon the flow rate. Accordingly, in practicing the methodof the invention, an operator will first draw a sample of air containinga known quantity of mercury through the test tube at a known flow rate.Then after determining the amount of mercury adsorbed by the goldcoating 22, and knowing the amount of air which has been passed throughthe test tube 10, the operator can calibrate the sample analyzing deviceaccordingly. Empirical tests have shown that measurements of a standardcan be reproduced very accurately with not more than plus or minus 6%error.

With one working model of the test tube 10 and-sampling device 30, atime period of 200 seconds was found to be very adequate for samplingair.The testv of course, an immediate analysis of the contents of thetest tube is to bemade. It is contemplated that the device 30 may beused at various locations far removed from analysis equipmenthereinafter to be described. Thus, it is contemplated that after thesample is collected in the gold coating 22 of a test tube 10, the tube.10 will be stoppered as shown in FIG. 4, and then sent,

such asby mail, to an analyzing station or laboratory.

dissolving the solid with an acid, diluting the dissolved solidacidmixture and placing the aqueous solution into test tube 10 and adding areducing agent to obtain free mercury. Likewise, for making an analysisof a liquid, one can simply place liquid in an aqueous solution intotest tube 10 and add a reducing agent. Empirical tests haveshown'thatalmost all of the mercury in the solution will be picked up bythe gold coating if the solution is agitated, such as by shaking, for atleast minutes. In this respect empirical tests have shown that shakingfor 20 minutesor more results in an adsorbtion of practically all of themercury in the liquid by the gold coating 22. Of course, when analyzinga liquid sample one would first make an analysis of a liquid having aknown concentration of mercury therein in order to properly calibratethe analyzing equipment hereinafter to be described. v V I Onesprocedurewhich worked very well for analyzing the mercury concentration in awater sample involved the placing of ten cubic centimeters of the watersample directly into the test tube and then adding 2 cubic centimetersof a 1.5 percent hydroxylamine hydrochloride solution. The'test tube 10was then stoppered and shaken for at least fifteen minutes.

When a liquid sample isbeing analyzed, the test tube' 10 is rinsed afterthe sample is removed therefrom and then dried before being-stopperedorbeforejmaking an analysis of the mercury picked up and adsorbed by thegold coating 22.

Since mercuryis vaporized from the gold coating 22 for ultravioletlight'absorption within the tube10, the tube 10 can be referred to as anabsorption cell inaddition to a sample. collecting tube. By combiningthe absorption cell into the collecting and sampling tube, the presentinvention obviates, if not altogether eliminates, the problems incurredwith previously proposed mercury analysis systems. In this respect,there is no transferring of a fluid sample from a collecting containeror tube to an absorption cell for analysis of the mercury concentrationin the fluid sample.

After each sample is made, the test tube will be given an identificationmarking or number to identify what was sampled, where and when. Thisinformation will preferably be in a code which can be sensed and thenprinted with the analysis of the mercury adsorbed by the gold coating22.

As previously described, the analysis involves driving out or vaporizingthe mercury from the gold coating 22 and then making an analysis of theamount of mercury vapor driven from the gold coating 22. This isaccomplished according to the method and system of the invention with avaporizing and analyzing apparatus shown in FIG. 5. The apparatus 80includes a housing 82 in which are mounted components of the apparatus80 including electronic circuit elements. The apparatus 80 also includesa test carrier 84 in which a plurality of test tubes 10 are carried in agenerally inverted upright position. The carrier is received in anopening 86 in the front side of the housing 82. Preferably the housing86 includes a suitable mechanism for indexing the carrier 84 into andthrough the housing 82 each time the analysis of the mercury in the goldcoating 22 in one of the test tubes 10 is completed whereby a pluralityof test tubes 10 can be analyzed automatically. Also the apparatus 80includes a printer 87 (FIG. 7) which provides a print-out tape 88 onwhich the analysis of the mercury adsorbed by the gold coating 22 ineach of the test tubes 10 is printed together with identifying codeinformation carried by each test tube 10.

A generally cylindrical device 90 for vaporizing mercury from the goldcoating 22 in each of the test tubes 10 carried by the carrier 84 issituated above the opening 86. The interior diameter of the device 90 isslightly larger than the outer diameter of the test tube 10, so that thedevice 90 can be easily received over the closed end 12 of the test tube10. Also, the device 90 is movablymounted in the opening 86 for movementfrom a raised position shown in FIG. 5 to a lowered position shown inFIG. 6 where the vaporizing device 90 is received over the portion ofone of the test tubes 10 extending upwardly from the carrier 84. 1

The vaporizing device 90 consists essentially of an infra-red heaterwhich includes a heating coil or element 94 imbedded in a material 96having high emissivity in the infrared spectral region. The vaporizingdevice or heater 90 is designed to rapidly heat the upper portion of theinverted test tube including the closed end 12 and the portion of thetube 10 which has the gold coating 22 on'the inner surface 20 thereof.It will be appreciated that the black coating on the exterior surface 24of the test tube 10 enhances the rapid heating of the portion of thetest tube 10 located within the heater 90. Also the black coatingprevents undesirable reflection, of the infra-red heat applied to thetest tube able in view of the heat-dissipation problems incurredtherewith. Very good results have been obtained however, by utilizing anelectric current through the heating element 94 which is sufficient toraise the temperature of the test tube 10 to approximately l000centigrade.

The rapid heating of the test tube 10 to a temperature on the order ofI,OOO centigrade produces very desirable results in that a thin film ofhot air and mercury vapor is quickly generated along the surface of thegold coating 22. This vapor rises in the inverted test tube 10 and formsa bubble of hot air and mercury vapor at the closed end 12 of the testtube 10. Although mercury vapor is heavier than air, the heated air andmercury vapor mixture is buoyant on the cooler air within the 9 testtube. As a result, the mercury vapor tends to remain at the upper end ofthe test tube 10 as shown in FIG. 6.

During the heating of the test tube 10, the heated gas within the upperpart of test tube 10 expands and it is desirable to allow the displacedcooler gas (air) to escape from the test tube 10. For this reason, thetest tube 10, when positioned underneath the heater 90, has the lowerend thereof in communication with a passageway 100 in a block 102 asshown in FIG. 6. The passageway 100 extends vertically downwardly into ablock 102 and then communicates with an outlet or vent passageway 104 inthe block 102. Typically, the passageway 104 vents to atmosphere.

From the foregoing description it will be understood that when a testtube is indexed to a position beneath the heater 90, the lower open end16 of the test tube 10 is in registry with the passageway 100 so thatthe test tube is vented. Then, when the upper portion of the invertedtest tube '10 is heated and the gases in the test tube expand, thecooler gas or air within the test tube is forced out of the test tube bythe expanding heated gases and mercury'vapor.

It will be appreciated that if the gold coating 22 on the interiorsurface 20 of the test tube 10 extended further toward the open end 16thereof, a greater amount of mercury vapor would be vaporized from thegold coating 22 into the test tube 10, and very possibly forced out ofthe test tube 10 through the vent passageway 104 by the heated expandinggases within the test tube 10. This possibility of losing some of themercury vapor creates a limitation on the length of the gold coating 22relative to the overall length of the test tube 10. As describedpreviously, a gold coating length to tube length'ratio of approximately0.40 has been found to work quite satisfactorily for obtaining asignificant sample of mercury none of which is lost during heating ofthe test tube 10.

After the mercury in the gold coating 22 has been vaporized, an analysisof the amount of mercury vapor to determine the concentration of mercuryin the sample from which the mercury vapor was originally obtained,

is made by passing ultraviolet light axially through the test tube 10and the mercury vapor therein. In the illustrated embodiment of theapparatus 80, this is accomplished with a light source 106 which directslight to a prism 108 mounted above the heater 90 in position to reflectlight downwardly through an opening 110 in the top of the heater 90. Theprism 108 is located to direct light downwardly along the axis of thetest tube 10.

The ultraviolet light from thesource 106 is in the 2537 angstrom bandwidth which is the band width of ultraviolet light that is absorbed bymercury vapor. To

minimize any optical aberrations which may be in curred as a result ofpassing the ultraviolet light through the heated closed end 12 of thetest tube 10 and the heated gases within the test tube 10, a quartzdiffuser 112 is situated between the source 106 and the prism 108 fordiffusing the light that is passed through the test tube 10. As shown inFIG. 6, the light passing through the test tube 10 passes through thepassageway 100 and thence through an ultraviolet band width pass filter114 to a photodetector 116. The filter 114 prevents any extraneous lightor radiant energy from reaching the photodetector 116 such as light fromthe heated gold coating 22. Typically the light source 106 iscontinuously on such that ultraviolet light is continuously passingthrough the test tube 10. The detector 116, however, is only switched onat certain times. In

carrying out the method, the apparatus is programmed to heat the testtube 10 for a predetermined period of time and then to switch on thephotodetector 116 which will immediately sense the amount of ultravioletlight passing through the mercury vapor in the test tube 10. Then, aftera measurement has been made of the ultraviolet light passed through thetest tube 10, the detector 116 will be turned off. Next, the mercuryvapor laden air is flushed from the test tube 10 with mercury-free air.This is accomplished by injecting mercury-free air into the test tube 10from a nozzle 118 mounted in the block 102 in position to direct airthere from upwardly along one side of the test tube 10 as best shown inFIG. 6. The flow of air through the nozzle 118 is controlled by a valveshown schematically at 120. Typically, the flushing air is ambient airwhich is passed through a filter 122. The filter 122 can containmanganese dioxide and/or activated charcoal.

After the mercury vapor laden air has been flushed from the test tube 10and only mercury free air is therein, the photodetector 116 is againactivated and another measurement of ultraviolet light reaching thephotodetector 116 is made. The difference between these measurements isindicative of the. amount of mercury vapor in the test tube 10. I

In FIG. 7 is illustrated a schematic block diagram of an electroniccircuit for comparing the light transmission measurements and thenconverting the difference to a signal value which is related to thenumerical value in parts per billion (PPB) of mercury in the fluidsamtained from ultraviolet light passing through the merv cury vaporladen air is identified as V and is stored in a first memory M Thesecond voltage signal derived from ultraviolet light passing throughmercuryfree air in the test tube 10, is designated V and is stored in amemory M The twovoltage signals V and V are then applied to a subtractorS A where the first voltage signal V is subtracted from the secondvoltage signal V The difference signal V -V is then applied to amultiplier 124. A stored :rnultiplicand which is determined from themeasurement of a fluid sample standard having a known concentration ofmercury therein is applied to the multiplier 124 for multiplying thedifference signal to obtain an output signal from the multiplier 124which is directly related to the numerical value of the concentration ofmercury in parts per billion. This signal is then applied to the printer97 which prints out the numerical value of the tape 88.

Ultraviolet light is also absorbed by some organic vapors and, to avoidinaccurate measurements of light intensity as may be occasioned by theabsorption of some of the ultraviolet light by organic vapors in thetest tube 10, it may be desirable to make measurements of ultravioletlight of a wavelength which is not absorbed by mercury. For this purposethe apparatus 80 of the mercury analysis system can include a secondlight source 130 as shown in FIG. 6. Also the electronic circuit can bemodified to include the additional memories M and M and an additionalsubtractor 8,, as shown in FIG. 8 connected between the subtractor S Aand the multiplier 124. The light from the source 130 is preferably of awavelength band which is near to the 2537 angstrom band but which doesnot include the 2537 angstrom wavelength which is absorbed by mercuryvapor.

air to obtain a second signal V which is stored in the memory M Thedifference V V, is determined by the subtractor S and the result storedin the memory M Then, after the mercury laden air has been flushed fromthe test tube 10 by mercury free air, a third signal V is obtained bypassing lightv from the source 106 through the mercury free air and thelogarithmic signal value V is'stored in the memory M,,. Then a fourthsignal V is obtained by passing light from the source 130 through themercury free air and stored in the memory M The difference V V isdetermined by the subtractor S and the result stored in the memory M Thetwo difference signals stored in the, memories M and M are then appliedto the subtractor 5;, where the second difference signal (V -V issubtracted from v necessary since in most cases the vapors in the tubeare oxidized by the heat from the infra-red heater 90 and the oxidizedorganic vapors generally do not absorb ultraviolet light in the mercuryabsorption band width of 2537 angstroms. g

- in F 10. 9 there is illustrated a modified mercury analysis system orapparatus 180 where a fixed cell or vessel 210 is utilized in place ofmovable tubes 10. The apparatus 180 is particularly useful foron-the-spot analysis of food (by thermal decomposition) or gases. The

' vessel 210 is positioned in a generally upright positionand has anupper end 212, a lower end 216,. an inner surface220 with a gold coatingor film 222 thereon and an infra-red absorbing, layer 224 on theoutersurface of the vessel 210. Agas input line 230 is connected to the upperend 212. A vacuum line 232 also is connected to the upper end 212. Asecond vacuum line 1234 and aclean air input line 236 are connected tothe lower end 216. An infrared heater 290 surrounds the vessel 210 inthe general area of the gold coating 222. A reflector 292 surrounds theheater 290 and has a quartz window 310 therein above the upper end 212.An ultraviolet light source 312 issituated above the quartz window.Beneath the lower end 216 is a 2537 angstrom band pass filter 314 and aphoto detector 316. Cooling air is supplied to the interior of thereflector through line 318.

' also minimizes, if not altogether eliminates, losses'and Each gas orair line has a suitable control valve therein and by programming theoperation ofthe valves, the heater 290 and the detector 316, mercuryfrom a given amount of a gas sample first can be concentrated in thegold coating 222 followed by the thermal release of mercury vapor fromthe coating 222 and buoyant maintenance of the vapor in the generallyvertical optical path between light source 312 and photodetector 316.Then a first light transmission measurement is made, the vessel 210 isflushed with mercuryfree air, a second light transmission measurement ismade and the measurements are compared to determine the concentration ofmercury in the gas sample.

It has been found that the mercury analysis system of the presentinvention utilizing the apparatus 80 or 180 enables an operator to makea mercury concentration analysis in a time period from three to fiveminutes which is a much faster time than obtained with many of thepreviously proposed mercury analysis systems. Also it has been foundthat the mercury analysis system of the present invention is verysensitive in that readings of 0.01 parts per billion (PPB) can beobtained therewith.

From the foregoing description it will be readily apparent that themercury analysis system and method of the present invention provide anumber of advantages some of which have been described above and othersof which are inherent in the invention. Some of these advantages are asfollows: l The system is very sensitive. (2) It is very flexible andenables one to make analysis of gases, liquids and solids such as foodstuffs. (3) The system is very simple and the simplicity is enhanced bycombining the sample collection tube and absorption cell into one testtube. (4) This simplicity cross-contamination. (5) The fluid samplingtime and the analysis time are very short, thereby facilitating theanalysis of numerous samples. (6) The system is very economical in thatone vaporizing and analyzing apparatus can be used for any number offield collecting test tubes 10. (7) The results obtained are readilyreproducible within a plus or minus 6 percent error.

From the foregoing description it will also be apparent thatmodifications and variations can be made to the test tube, method andsystem of the present invention without departing from the spirit orscope of the invention. Accordingly, the present invention is only to belimited as necessitated by the accompanying claims.

What it is desired to be secured by Letters Patent of the United Statesis:

l. A method for analyzing the concentration of mercury in a fluidincluding the steps of: placing a given amount of a fluidsample in avessel having a predetermined interior side surface area coated with anoble metal which has a strong affinity for mercury, such that at leasta portion of the mercury in said sample is adsorbed by said noble metalcoating; removing said sample from said vessel; positioning said vesselin a generally upright position with a first end of said vessel above asecond end of said vessel and with said second end vented; heating saidvessel to a given temperature to release mercury vapor from said noblemetal coating; passing endwise through said vessel an ultraviolet lightof a wave length band which is absorbed by mercury vapor; measuring theamount of said light transmitted through the mercury vapor laden airwithin said vessel; flushing said mercury vapor laden air from saidvessel with mercury-free air; passing said ultraviolet light endwisethrough said vessel having mercury-free air therein; measuring theamount of said light transmitted through said mercury-free air; andcomparing the first light transmission measurement with the second lighttransmission measurement, the difference between said measurements beingindicative of the concentration of mercury in said fluid sample.

2. The method according to claim 1 wherein said fluid is a gas and saidsteps of placing sample and removing sample from said vessel consists inpassing said gas through said vessel at a given flow rate for a giventime period.

3. The method according to claim 2 wherein said gas is injected intosaid vessel at a point near said first end of said vessel so that saidgas flows along and adjacent the interior noble-metal-coated surface ofsaid vessel and out said second end of said vessel.

4. The method according to claim 2 wherein said gas is injected intosaid vessel through a pipe which extends into said vessel at said secondend and opens into said vessel adjacent said first end and said gas iscaused to flow through said pipe into said vessel along and adjacent theinterior noble-metal-coated, surface of said vessel and out said secondend of said vessel by applying a vacuum to said second end of saidvessel.

5. The method according to claim 2 wherein said vessel is positioned ina generally upright position while said gas is being passed through saidvessel.

6. The method according to claim 1 wherein said fluid sample is anaqueous solution and said method includes the steps of adding a reducingagent to the given amount of aqueous solution in said vessel; sealingsaid vessel; and agitating said aqueous solution and reducing agent fora given period of time to facilitate reduction of mercury compounds tofree metal mercury and to facilitate adsorption of said mercury by saidnoble metal coating.

7. The method according to claim 6 wherein said aqueous solution sampleincludes dissolved solids and said method includes the initial stepsof;dissolving a solid in an acid and diluting the acid-dissolved solidwith water to form the aqueous solution sample.

8. The method according to claim 7 wherein said solid is a foodstuff. a

9. The method according to claim 6 wherein said step of agitating saidsolution consists essentially in shaking said vessel.

10. The method according to claim 6 wherein said aqueous solution isagitated for at least five minutes.

11. The method according to claim 6 including the steps of rinsing anddrying said vessel prior to heating said vessel.

12. The method according to claim 6wherein said sample consistsessentially of approximately ten cubic centimeters of water and saidreducing agent consists essentially of approximately two cubiccentimeters of an approximately 1.5 percent hydroxylamine hydrochlorideaqueous solution.

13. The method according to claim 1 wherein said fluid sample is a gassample including vapors from a thermally decomposed solid and saidmethod includes the initial step of thermally decomposing a solid toobtain vapors which form said fluid sample.

14. The method according to claim 1 wherein said noble metal is gold. Ig

15. The method according to claim 1 wherein said vessel is heated to atemperature between 400C and l,300C.

16. The method according to claim 1 wherein said vessel is heated byinfra-red radiation.

17. The method according to claim 2 including the initial step offiltering said gas to remove undesirable particles therefrom.

18. The method according to claim 1 wherein said ultraviolet light isdiffused before being passed endwise through said vessel.

19. The method according to claim 1 including the step of preheatingsaid fluid sample to vaporize mercury in any solid particles carried insaid sample.

20. The method according to claim 1 including the steps of amplifyingand logarithmically converting the light measurements to signal valueswhich are directly related to the concentration of mercury in said fluidsample. 1

21. The method according to claim 20 including the steps of subtractingsaid first signal value from said second signal value; multiplying thedifference by a predetermined multiplicand to obtain a numerical valuein parts per billion; and printing said numerical value.

22. The method according to claim 1 including the steps of: passingthrough said mercury vapor laden air ultraviolet light of a secondwavelength band which is near to, but which does not include, saidmercury absorption band; measuring the amount of said light having saidsecond wave' length which is transmitted through said mercuryvapor ladenair to obtain a light transmission measurement; passing said lighthaving said second wave length band through said mercuryfree air;measuring the amount of said light having said second wave length bandwhich is transmitted through said mercury-free air to obtain a lighttransmission measurement; amplifying and logarithmically converting saidlight transmission measurements to signal values proportional to thelogarithm of said light transmission measurements; subtracting saidfirst signal value derived from the measurement of transmission throughsaid mercury vapor laden air of ultraviolet light of a wavelength bandwhich is absorbed by mercury vapor, from said signal derived from themeasurement of transmission through said mercury vapor laden air ofultraviolet light of said second wave length band to obtain a firstdifference signal; subtracting said signal value derived from themeasurement of transmission through said mercury-free air of ultravioletlight of said wave length band which is absorbed by mercury vapor fromsaid signal derived from the measurement of transmission through saidmercury-free air of ultraviolet light of said second wave length band toobtain a second difference signal; and subtracting said seconddifference signal from said first difference signal to obtain a signalvalue which is related to the concentration of mercury in said fluidsample and which has had subtracted therefrom the undesired effects ofultraviolet light absorption by other vapors which may be in said vesselin addition to mercury vapor.

23. The method according to claim 22 including the steps of multiplyingsaid last signal value by a given multiplicand to obtain a numericalvalue in parts per billion; and printing said numerical value.

24. A mercury analysis system including a vessel having a noblemetal-coating on an interior side surface thereof and being supported ina generally upright position with an upper end of the vessel above alower end and with the lower end of the vessel vented, means forvaporizing mercury stored in said noble metal coating, and means foranalyzing the mercury vapor, said analyzing means including a source ofultraviolet light of a wave length band which is absorbed by mercuryvapor, said source of ultraviolet light being positioned to passultraviolet light endwise through the vessel, means for detecting theamount of light transmitted through the vessel, and means for flushingmercury vapor laden air from the vessel.

25. The system according to claim 24 including an electronic circuitconnected to said detecting means, said electronic circuit includingmeans for amplifying signals generated by light detected by saiddetecting means and for logarithmically converting the generated signalsto signal values related to the concentration of mercury in the mercuryvapor laden air, means for obtain difference signal directly related tothe concentration of mercury in the mercury vapor laden air.

26. The system accordingto claim 25 includingmeans for printing themercury analysis measurement in parts per billion and wherein saidelectronic circuit includesa multiplying means connected between said27. The system according to claim 25 including a sec ond sourceofuitraviolet light of a wavelength band which is near, but which doesnot include, the mercury vapor absorption band, said second source beingpositio ned to pass ultraviolet light endwise. through the vessel atpredetermined times, and wherein a memorizing means is operable tomemorize a generated signal obtained when ultraviolet light from saidsecond source is passed through the mercury vapor laden air in thevessel; a memory means is operable to memorize a generatedsignalobtained when ultraviolet light from said second source is passedthrough mercury-free air in the vessel, said subtracting means beingoperable to subtract. said first signal obtained when ultraviolet lightfrom said first source is passed through the mercury vapor laden air inthe vessel from said signal obtained when ultraviolet light from saidsecond source is passed through the mercury vapor laden air in thevessel to obtain a first difference signal, memory means operable tomemorize a third signal obtained when ultraviolet light from said firstsource is passed through mercuryfree air in the vessel, memory meansoperable to memorize a fourth signal obtained when ultraviolet lightfrom said second source is passed through mercuryfree air in the vessel,said subtracting means being operable to subtract said third signal fromsaid fourth signal to obtain a second difference signal, and saidelectronic circuit includes a means for memorizing said first differencesignal and a means for memorizing said second difference signal and asecond subtracting means for subtracting said second difference signalfrom said first difference signal to obtain a third difference signalwhich is directly related to mercury vapor vaporized in the test tubeand which has had subtracted therefrom the undesired effects ofultraviolet light absorption by other vapors which may be in the vesselin addition to mercury vapor.

28. The system according to claim 27 including means for printing themercury analysis of a fluid sample in parts per billion and wherein saidelectronic circuit includes multiplying means connected between saidsecond subtracting means and said printing means for multiplying saidthird difference signal by a predetermined multiplicand to obtain anumerical value in parts per billion of the mercury concentration in thefluid sample. i

29. The system according to claim 24 wherein said flushing meansincludes a filter for removing mercury vapors from air used as aflushing medium, and a flush nozzle having its outlet orifice arrangedto inject flushing air into the lower'end of the vessel adjacent oneside of the vessel and in a direction generally parallel to thelongitudinal axis of the vessel. a

l 30. The system according to claim 24 including an ultraviolet bandpassfilter situated between one end of the vessel and said detecting means.

31. The system according to claim 24 wherein said vaporizing means is aninfra-red heater.

32. The system according to claim 24 wherein said light source ishorizontally spaced from the upper end of the vessel and a prism issituated above the upper 1 end of the vessel for reflecting light fromsaid light source downwardly through vessel. v

33. The system according to claim 24 including means interposed betweensaid light source and the vessel for diffusing the light from said lightsource.

1. A method for analyzing the concentration of mercury in a fluidincluding the steps of: placing a given amount of a fluid sample in avessel having a predetermined interior side surface area coated with anoble metal which has a strong affinity for mercury, such that at leasta portion of the mercury in said sample is adsorbed by said noble metalcoating; removing said sample from said vessel; positioning said vesselin a generally upright position with a first end of said vessel above asecond end of said vessel and with said second end vented; heatinG saidvessel to a given temperature to release mercury vapor from said noblemetal coating; passing endwise through said vessel an ultraviolet lightof a wave length band which is absorbed by mercury vapor; measuring theamount of said light transmitted through the mercury vapor laden airwithin said vessel; flushing said mercury vapor laden air from saidvessel with mercury-free air; passing said ultraviolet light endwisethrough said vessel having mercury-free air therein; measuring theamount of said light transmitted through said mercury-free air; andcomparing the first light transmission measurement with the second lighttransmission measurement, the difference between said measurements beingindicative of the concentration of mercury in said fluid sample.
 2. Themethod according to claim 1 wherein said fluid is a gas and said stepsof placing sample and removing sample from said vessel consists inpassing said gas through said vessel at a given flow rate for a giventime period.
 3. The method according to claim 2 wherein said gas isinjected into said vessel at a point near said first end of said vesselso that said gas flows along and adjacent the interiornoble-metal-coated surface of said vessel and out said second end ofsaid vessel.
 4. The method according to claim 2 wherein said gas isinjected into said vessel through a pipe which extends into said vesselat said second end and opens into said vessel adjacent said first endand said gas is caused to flow through said pipe into said vessel alongand adjacent the interior noble-metal-coated surface of said vessel andout said second end of said vessel by applying a vacuum to said secondend of said vessel.
 5. The method according to claim 2 wherein saidvessel is positioned in a generally upright position while said gas isbeing passed through said vessel.
 6. The method according to claim 1wherein said fluid sample is an aqueous solution and said methodincludes the steps of adding a reducing agent to the given amount ofaqueous solution in said vessel; sealing said vessel; and agitating saidaqueous solution and reducing agent for a given period of time tofacilitate reduction of mercury compounds to free metal mercury and tofacilitate adsorption of said mercury by said noble metal coating. 7.The method according to claim 6 wherein said aqueous solution sampleincludes dissolved solids and said method includes the initial steps of;dissolving a solid in an acid and diluting the acid-dissolved solid withwater to form the aqueous solution sample.
 8. The method according toclaim 7 wherein said solid is a foodstuff.
 9. The method according toclaim 6 wherein said step of agitating said solution consistsessentially in shaking said vessel.
 10. The method according to claim 6wherein said aqueous solution is agitated for at least five minutes. 11.The method according to claim 6 including the steps of rinsing anddrying said vessel prior to heating said vessel.
 12. The methodaccording to claim 6 wherein said sample consists essentially ofapproximately ten cubic centimeters of water and said reducing agentconsists essentially of approximately two cubic centimeters of anapproximately 1.5 percent hydroxylamine hydrochloride aqueous solution.13. The method according to claim 1 wherein said fluid sample is a gassample including vapors from a thermally decomposed solid and saidmethod includes the initial step of thermally decomposing a solid toobtain vapors which form said fluid sample.
 14. The method according toclaim 1 wherein said noble metal is gold.
 15. The method according toclaim 1 wherein said vessel is heated to a temperature between 400*C and1,300*C.
 16. The method according to claim 1 wherein said vessel isheated by infra-red radiation.
 17. The method according to claim 2including the initial step of filtering said gas to remove undesirableparticles therefrom.
 18. The method according to claim 1 wherein Saidultraviolet light is diffused before being passed endwise through saidvessel.
 19. The method according to claim 1 including the step ofpreheating said fluid sample to vaporize mercury in any solid particlescarried in said sample.
 20. The method according to claim 1 includingthe steps of amplifying and logarithmically converting the lightmeasurements to signal values which are directly related to theconcentration of mercury in said fluid sample.
 21. The method accordingto claim 20 including the steps of subtracting said first signal valuefrom said second signal value; multiplying the difference by apredetermined multiplicand to obtain a numerical value in parts perbillion; and printing said numerical value.
 22. The method according toclaim 1 including the steps of: passing through said mercury vapor ladenair ultraviolet light of a second wavelength band which is near to, butwhich does not include, said mercury absorption band; measuring theamount of said light having said second wave length which is transmittedthrough said mercury vapor laden air to obtain a light transmissionmeasurement; passing said light having said second wave length bandthrough said mercury-free air; measuring the amount of said light havingsaid second wave length band which is transmitted through saidmercury-free air to obtain a light transmission measurement; amplifyingand logarithmically converting said light transmission measurements tosignal values proportional to the logarithm of said light transmissionmeasurements; subtracting said first signal value derived from themeasurement of transmission through said mercury vapor laden air ofultraviolet light of a wavelength band which is absorbed by mercuryvapor, from said signal derived from the measurement of transmissionthrough said mercury vapor laden air of ultraviolet light of said secondwave length band to obtain a first difference signal; subtracting saidsignal value derived from the measurement of transmission through saidmercury-free air of ultraviolet light of said wave length band which isabsorbed by mercury vapor from said signal derived from the measurementof transmission through said mercury-free air of ultraviolet light ofsaid second wave length band to obtain a second difference signal; andsubtracting said second difference signal from said first differencesignal to obtain a signal value which is related to the concentration ofmercury in said fluid sample and which has had subtracted therefrom theundesired effects of ultraviolet light absorption by other vapors whichmay be in said vessel in addition to mercury vapor.
 23. The methodaccording to claim 22 including the steps of multiplying said lastsignal value by a given multiplicand to obtain a numerical value inparts per billion; and printing said numerical value.
 24. A mercuryanalysis system including a vessel having a noble metal coating on aninterior side surface thereof and being supported in a generally uprightposition with an upper end of the vessel above a lower end and with thelower end of the vessel vented, means for vaporizing mercury stored insaid noble metal coating, and means for analyzing the mercury vapor,said analyzing means including a source of ultraviolet light of a wavelength band which is absorbed by mercury vapor, said source ofultraviolet light being positioned to pass ultraviolet light endwisethrough the vessel, means for detecting the amount of light transmittedthrough the vessel, and means for flushing mercury vapor laden air fromthe vessel.
 25. The system according to claim 24 including an electroniccircuit connected to said detecting means, said electronic circuitincluding means for amplifying signals generated by light detected bysaid detecting means and for logarithmically converting the generatedsignals to signal values related to the concentration of mercury in themercury vapor laden air, means for memorizing a first generated signalrelated to the light transmitted throUgh mercury-laden-vapor, means formemorizing a second generated signal related to the light transmittedthrough mercury-free air, and means for subtracting the first signalfrom the second signal to obtain difference signal directly related tothe concentration of mercury in the mercury vapor laden air.
 26. Thesystem according to claim 25 including means for printing the mercuryanalysis measurement in parts per billion and wherein said electroniccircuit includes a multiplying means connected between said subtractingmeans and said printing means for multiplying the difference signal by apredetermined multiplicand.
 27. The system according to claim 25including a second source of ultraviolet light of a wavelength bandwhich is near, but which does not include, the mercury vapor absorptionband, said second source being positioned to pass ultraviolet lightendwise through the vessel at predetermined times, and wherein amemorizing means is operable to memorize a generated signal obtainedwhen ultraviolet light from said second source is passed through themercury vapor laden air in the vessel; a memory means is operable tomemorize a generated signal obtained when ultraviolet light from saidsecond source is passed through mercury-free air in the vessel, saidsubtracting means being operable to subtract said first signal obtainedwhen ultraviolet light from said first source is passed through themercury vapor laden air in the vessel from said signal obtained whenultraviolet light from said second source is passed through the mercuryvapor laden air in the vessel to obtain a first difference signal,memory means operable to memorize a third signal obtained whenultraviolet light from said first source is passed through mercury-freeair in the vessel, memory means operable to memorize a fourth signalobtained when ultraviolet light from said second source is passedthrough mercury-free air in the vessel, said subtracting means beingoperable to subtract said third signal from said fourth signal to obtaina second difference signal, and said electronic circuit includes a meansfor memorizing said first difference signal and a means for memorizingsaid second difference signal and a second subtracting means forsubtracting said second difference signal from said first differencesignal to obtain a third difference signal which is directly related tomercury vapor vaporized in the test tube and which has had subtractedtherefrom the undesired effects of ultraviolet light absorption by othervapors which may be in the vessel in addition to mercury vapor.
 28. Thesystem according to claim 27 including means for printing the mercuryanalysis of a fluid sample in parts per billion and wherein saidelectronic circuit includes multiplying means connected between saidsecond subtracting means and said printing means for multiplying saidthird difference signal by a predetermined multiplicand to obtain anumerical value in parts per billion of the mercury concentration in thefluid sample.
 29. The system according to claim 24 wherein said flushingmeans includes a filter for removing mercury vapors from air used as aflushing medium, and a flush nozzle having its outlet orifice arrangedto inject flushing air into the lower end of the vessel adjacent oneside of the vessel and in a direction generally parallel to thelongitudinal axis of the vessel.
 30. The system according to claim 24including an ultraviolet band pass filter situated between one end ofthe vessel and said detecting means.
 31. The system according to claim24 wherein said vaporizing means is an infra-red heater.
 32. The systemaccording to claim 24 wherein said light source is horizontally spacedfrom the upper end of the vessel and a prism is situated above the upperend of the vessel for reflecting light from said light source downwardlythrough vessel.
 33. The system according to claim 24 including meansinterposed between said light source and the vessel for diffusing thelight frOm said light source.